Ohnesorge et al, U.S. Pat. No. 4,422,335 discloses a pressure transducer made using quartz plates and Bernot, U.S. Pat. No. 5,189,591 discloses a pressure transducer using alumino-silicate glass plates. Whether glass or quartz, these sensors have generally been mounted to headers made of aluminum or stainless steel. Besides being expensive to machine, another disadvantage to these aluminum and stainless steel headers is that it is difficult to hermetically seal the electrical connections to these sensors. Without a hermetic seal, liquid and gas contaminants in the sensing medium can enter and ruin the sensor. As a result these types of sensors can only be used where the pressure medium is contaminant free such as dry gasses.
A further advance in sensor technology is disclosed in Bernot et al, U.S. patent application Ser. No. 08/986,253, filed Dec. 5, 1997, entitled, (as amended), "Capacitive Pressure Sensor Housing Having a Ceramic Base" which is assigned to the assignee of this application. This application discloses a sensor in which the prior art aluminum or stainless steel headers, (also referred to as bases), are replaced with a ceramic header and a cover that is either ceramic or metallic. The cover and header are sealed together with glass frit to define an interior chamber. A capacitive pressure sensor with alumino-silicate glass plates is mounted within the chamber.
Referring to FIGS. 1-5, in the fabrication of low-cost pressure sensor a critical component is the sensor base or header 36 that supports the housing 24 and the sensor 10 which is frit bonded to the top surface of pillar 50. The base can be fabricated by a number low cost ceramic process which form the base from a glass powder. Because of the relatively small dimensions of the pillars 50, it is difficult with these processes to achieve the required surface flatness of the top surfaces of the pillars 50 without the additional steps machining and grinding the surface. These steps of course increase manufacturing cost and reduce yield. They also cause a wide variation in tolerance that reduces sensor accuracy.
Accordingly, there is a need for base or header design that eliminates the flatness problem associated with the pillars.